Data storage system with redundant media handling assemblies

ABSTRACT

A data storage system comprising: a media picker displacement path having a first end and a second end; a first media picker assembly mounted in the picker displacement path; a second media picker assembly mounted in the picker displacement path; a plurality of data storage media access locations located along the picker displacement path; the first media picker assembly being displaceable along the path from the first end to a point of interfering contact with the second media picker assembly; the second media picker assembly being displaceable along the path from the second end to a point of interfering contact with the first media picker assembly; the storage system having a normal running operating mode wherein the first media picker assembly is stationarily positioned at the first end of the displacement path and the second picker assembly is selectively positionable along the remainder of the path; the storage system having a fall back running operating mode wherein the second media picker assembly is stationarily positioned at the second end of the displacement path and the first picker assembly is selectively positionable along the remainder of the path.

[0001] This application is a continuation of U.S. application, Ser. No.09/311, 243 filed on May 13, 1999, which is hereby incorporated byreference for all that is disclosed therein.

FIELD OF THE INVENTION

[0002] The present invention relates generally to data storage systemsfor handling and storing data storage media devices such as datacartridges, and more particularly to a data storage system havingredundant media handling assemblies.

BACKGROUND OF THE INVENTION

[0003] Data storage in the computer industry is accomplished in a numberof ways. For example, data may be stored on various data storage mediadevices such as tapes, compact disks, “floppy” or “hard ” disks, and thelike. Oftentimes, data storage media which is transferrable from onelocation to another is housed within a parallelepiped-shaped cartridge.It is to be understood that the term “cartridge ” or “data cartridge ”as used in the present application encompasses any data storage mediadevice, whether or not it is housed within a cartridge.

[0004] Data storage systems are used to store data cartridges at knownlocations and to retrieve desired cartridges so that data may be writtento or read from the cartridges. A typical data storage system mayinclude different types of cartridge receiving devices. For example, onetype of cartridge receiving device is a cartridge storage rackor“magazine ” which has a plurality of individual cartridge storagelocations. Another type of cartridge receiving device is a cartridgeread/write device or “drive ”. A data storage system may also include acartridge handling assembly for retrieving data cartridges andtranslating them among cartridge receiving devices (e.g., from acartridge storage location to a drive and vice-versa).

[0005] Data storage systems may be produced in a variety of sizes andconfigurations. One type of data storage system has a guide trackextending along the length of a central vertical shaft. A single datacartridge handling assembly or“picker ” assembly is verticallydisplaceable along the track and is adapted to move data cartridgesbetween cartridge receiving devices positioned adjacent to the centralvertical shaft in vertically stacked layers or tiers. In some storagesystems of this type all components are provided in a single housingunit. In other storage systems of this type a plurality of modularhousing units are stacked one on top of the other to provide a compoundhousing assembly. In a compound housing assembly each modular housingunit contains a portion of the vertical shaft and one or more of thetiers of cartridge receiving devices of the data storage system. Such anassembly of modular units is disclosed in U.S. Patent Application Ser.No. 09/137,350 filed Aug. 20, 1998 for MODULAR DATA STORAGE SYSTEM ofJoseph M. White, Matthias Lester, and Dave Jones, which is herebyincorporated for all that it discloses. An advantage of an integral datastorage system having multiple vertically stacked tiers accessible by asingle picker assembly over systems having multiple independent singletier units, each with its own picker assembly, is that it obviates theneed to coordinate the operation of multiple systems. Also, unnecessaryduplication of certain system components, such as drives and pickers, isavoided. However one disadvantage of a unitary system having a singlepicker assembly is that a failure of the picker assembly causes loss ofaccess to all of the media in the storage system until the picker isrepaired or replaced. It would be generally desirable to provide a datastorage system having multiple vertically stacked tiers accessible by asingle picker assembly with a subsystem which would automaticallyreplace a nonoperating picker assembly with an operating picker assemblyso that the user's access to data stored in the system is notinterrupted.

SUMMARY OF THE INVENTION

[0006] The present invention is directed to a data storage system forhandling and storing data storage media. The system ordinarily uses asingle main media picker assembly to move media between media receivingdevices positioned along a picker displacement path. If the main pickerassembly fails it is automatically removed from a portion of the pathassociated with media access and is replaced by a “fall back”/“backup”/“redundant ” picker assembly which continues operation of the datastorage system without significant interruption.

[0007] Thus the invention may comprise a data storage system comprising:a housing assembly having a first end and a second end and having anopen shaft extending between said first end and said second end andcomprising: a first housing unit having a first portion of said openshaft extending therethrough, said first unit being positioned proximatesaid first end of said housing assembly; a second housing unit having asecond portion of said open shaft extending therethrough, said secondunit being positioned proximate said second end of said housingassembly; at least a third housing unit having a third portion of saidopen shaft extending therethrough having a plurality of data mediareceiving devices mounted therein, said third housing unit beingdisposed between said first and second housing units; a first mediahandling assembly displaceably positioned within said open shaft; and asecond media handling assembly displaceably positioned within said openshaft.

[0008] The invention may also comprise a data storage system comprising:a media picker displacement path having a first end and a second end; afirst media picker assembly mounted in the picker displacement path; asecond media picker assembly mounted in the picker displacement path; aplurality of data storage media access locations located along thepicker displacement path; the first media picker assembly beingdisplaceable along the path from the first end to a point of interferingcontact with the second media picker assembly; the second media pickerassembly being displaceable along the path from the second end to apoint of interfering contact with the first media picker assembly.

[0009] The invention may also comprise a method of operating a datastorage library comprising: moving data storage media between mediaholding positions located along a one-picker-at-a-time pickerdisplacement path with a first media picker assembly until the firstmedia picker assembly malfunctions; after it malfunctions moving thefirst picker assembly to a first end of the picker displacement path;maintaining a second media picker assembly at a stationary position at asecond end portion of the picker displacement path until the firstpicker assembly malfunctions; after the first picker assemblymalfunctions moving data storage media between media holding positionslocated along the picker displacement path with the second pickerassembly.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Illustrative and presently preferred embodiments of the inventionare shown in the accompanying drawings in which:

[0011]FIG. 1 is an isometric view of a modular data storage system;

[0012]FIG. 2 is an isometric view of a modular unit from the modulardata storage system of FIG. 1, with most of the housing broken away;

[0013]FIG. 3 is a top plan view of the modular unit of FIG. 2schematically illustrating a preferred mounting position of the actuatorsystem and pinion gear assembly;

[0014]FIG. 4 schematically illustrates an alternative longitudinalmounting position for the actuator system and pinion gear assembly ofFIG. 3;

[0015]FIG. 5 schematically illustrates an alternative lateral mountingposition for the actuator system and pinion gear assembly of FIG. 3;

[0016]FIG. 6 is an enlarged side elevational view schematicallyillustrating an upper rack and a lower rack from two adjacent modularunits and a pinion gear of the modular data storage system of FIG. 1;and

[0017]FIG. 7 is an isometric view of modular units from the modular datastorage system of FIG. 1 mounted within a rack mount type housing.

[0018]FIG. 8 is a schematic cross sectional elevation view of a datastorage system in a shipping/pre-startup mode.

[0019]FIG. 9 is a schematic cross sectional elevation view of a datastorage system in a normal operating mode.

[0020]FIG. 10 is schematic cross sectional elevation view of a datastorage system in a fall back operating mode.

[0021]FIG. 11 is a block diagram of a data storage system centralcontrol unit and two cartridge handling assemblies.

DETAILED DESCRIPTION OF THE INVENTION

[0022] The drawing, in general, illustrates a data storage system (10)comprising: a media picker displacement path (19) having a first end(17) and a second end (21); a first media picker assembly (50) mountedin the picker displacement path (19); a second media picker assembly(51) mounted in the picker displacement path (19); a plurality of datastorage media access locations (40) located along the pickerdisplacement path (19); the first media picker assembly (50) beingdisplaceable along the path from the first end (17) to a point ofinterfering contact with the second media picker assembly (51); thesecond media picker assembly (51) being displaceable along the path (19)from the second end (21) to a point of interfering contact with thefirst media picker assembly (50).

[0023] Having thus generally described a data storage system 10, variousembodiments thereof will now be described in detail. One embodiment of amodular data storage system 10 is shown in FIG. 1 and comprises at leastthree modular units 12, 13, 14. The modular units 12, 13, 14 arestackable vertically upon one another to form a compound housingassembly. While three modular units are shown in FIG. 1, it is to beunderstood that as few as three, or as many modular units as a customerdesires, may be utilized in the modular data storage system 10.Furthermore, one or more modular units may be added to the modular datastorage system at any time. The modular data storage system 10 may alsocomprise cartridge (media) handling assemblies 50, 51 (generallyreferred to in the art as “picker assemblies” or “pickers”) which may bevertically translated along an open shaft 19 extending through theadjacent modular units 12, 13, 14 and having a first end 17 and a secondend 19, as described in further detail below.

[0024] In one embodiment the modular units 12, 13, 14 are substantiallyidentical to one another, except where noted otherwise below. Eachmodular unit 12, 13, 14 may comprise a housing 30, 32, 34, each havingan upper opening, e.g., 36 and a lower opening, e.g., 38. Each of theopenings 36, 38 are sized and shaped to allow the cartridge handlingassembly 50 to pass therethrough. Each intermediately positioned modularunit 13, etc., i.e. all modular units except for the top most and bottommost units 12, 14 may further comprise a plurality of cartridgereceiving devices 40 which may be, for example, magazines (e.g., 42) ordrives (e.g., 44), or any combination thereof. The configuration ofcartridge receiving devices 40 within each intermediate modular unit 13need not be identical; each intermediate modular unit may be configuredto suit a customer's particular needs.

[0025] As described elsewhere herein the top and bottom modules 12, 14serve primarily as storage and access bays for the currently inactivepicker assembly 50, 51. Thus, the top and bottom modules 12, 14 may beidentical to the intermediate module(s) 13, etc. except that nocartridge receiving devices 40 would ordinarily be positioned therein.

[0026] The cartridge handling assembly 50 may be of the type describedin U.S. patent application Ser. No. 09/045,134 filed 3/20/98 forMULTIPLANE TRANSLATING CARTRIDGE HANDLING SYSTEM of Gregg Schmidtke andRobert Mueller, now U.S. Pat. No. 6,025,972, issued Feb. 15, 2000, whichis hereby incorporated by reference for all that is contained therein.FIG. 2 illustrates a modular unit 12 with most of the housing 30 brokenaway to more clearly show the cartridge handling assembly 50. It is tobe understood that the description below, except for the absence ofcartridge receiving devices 40 in units 12 and 14, may apply to all ofthe modular units (e.g., 12, 13, 14) of the modular data storage system10.

[0027] As shown in FIG. 2, the cartridge handling (picker) assembly 50may comprise a generally rectangular frame 16 having an upper portion18, a lower portion 20, and four sides 22, 24, 26, 28. Cartridgereceiving devices 40 (magazines 42 and drives 44) are preferablyaccessible from at least one, and most preferably two or more, of thesides 22, 24, 26, 28. The cartridge handling assembly 50 may alsocomprise a cartridge access device 52 capable of loading and retrievingthe data cartridges (not shown) from the cartridge receiving devices 40.Cartridge handling assembly 51 may be identical to cartridge handlingassembly 50. Z-fold ribbon cable (not shown) may be attached at one endto the top of the cartridge handling assembly 50 and extended out thetop of the shaft 19 and attached at the other end to a control unit 400,FIG. 11. A second Z-fold cable may be attached to the lower handlingassembly 51 and extended out the bottom of shaft 19 to connect it to thecontrol unit.

[0028] The modular data storage system 10 may also comprise a verticallift system 58, which is best shown in FIG. 2. The vertical lift system58 may comprise a pinion gear assembly 60 mounted on the cartridgehandling assembly 50. The vertical lift system 58 may further comprisean actuator system 70 operably attached to the pinion gear assembly 60for vertically translating the cartridge handling assembly 50 among theadjacent modular units 12, 13, 14 (FIG. 1). The vertical lift system 58may further comprise a rack assembly (e.g., 80) mounted on each of saidmodular units (e.g., 12), which is operably associated with the piniongear assembly 60.

[0029] The pinion gear assembly 60 is preferably comprised of a firstpinion gear 62 and a second pinion gear 64, mounted diagonally oppositeone another as shown in FIG. 2. Each pinion gear 62, 64 is preferably atleast partially enclosed within a protective housing 66, 68,respectively. The rack assembly 80 as shown in FIG. 2 is preferablycomprised of a first rack 82 and a second rack 84, mounted diagonallyopposite one another and in alignment with the pinion gears 62, 64,respectively. Each rack 82, 84 is fixedly attached to the modular unithousing 30. The actuator system 70 is preferably comprised of a motor72, a gear/shaft assembly 74 to operably attach the motor 72 to thepinion gears 62, 64, and a protective housing 76. The actuator systemprotective housing 76 may be integrally formed with at least one of thepinion gear protective housings (e.g. 66), as shown in FIG. 2.

[0030] The actuator system 70 and pinion gears 62, 64 of the pinion gearassembly 60 are preferably fixedly attached to the cartridge handlingassembly frame 16 at the upper portion 18 thereof. The actuator system70 and pinion gears 62, 64 are preferably mounted diagonally as shown inFIGS. 1, 2 and schematically in FIG. 3 in order to most evenlydistribute the weight thereof along the upper portion 18 of the frame16. The racks 82, 84 are also mounted diagonally as shown in thisembodiment. FIGS. 4 and 5 illustrate alternate mounting positions forthe actuator system, pinion gears and racks. As shown in FIG. 4, theactuator system 70′ may be mounted longitudinally, with the pinion gears62′, 64′ being mounted longitudinally opposite one another and the racks82′, 84′ also being mounted longitudinally opposite one another. Asshown in FIG. 5, the actuator system 70″ may be mounted laterally, withthe pinion gears 62″, 64″ being mounted laterally opposite one anotherand the racks 82″, 84″ also being mounted laterally opposite oneanother. Furthermore, the actuator system 70 may be mounted diagonally,longitudinally or laterally as shown in FIGS. 3-5 at either the upperportion 18 or the lower portion 20 (FIG. 2) of the frame 16.

[0031] Referring again to FIG. 1, among adjacent modular units 12, 13,14 there is an upper modular unit (e.g., 12) and a lower modular unit(e.g. 13). The upper modular unit 12 comprises an upper rack assembly80, and the lower modular unit comprises a lower rack assembly 90. FIG.6 illustrates an upper rack 82 and a lower rack 92 from two adjacentmodular units 12, 13, respectively, and a pinion gear 62. It is to beunderstood that the following description also applies to the secondrack (e.g., 84) of each rack assembly 80, 90 and also to the secondpinion gear 64 of the pinion gear assembly 60.

[0032] As shown in FIG. 6, the upper rack 82 comprises a plurality ofgear teeth 100 having a predetermined pitch “P” (measuredcenterline-to-centerline), and the lower rack 92 also comprises aplurality of gear teeth 110 having the same predetermined pitch “P”. Theupper rack 82 comprises a lowermost gear tooth 102, and the lower rack92 comprises an uppermost gear tooth 112. In order for the pinion gear62 to translate smoothly between the upper rack 82 and lower rack 92,two conditions must exist. First, the adjacent modular units 12, 13 mustbe in substantial alignment. More specifically, the upper rack 82 of theupper modular unit 12 and the lower rack 92 of the lower modular unit 13must be in substantial alignment (and, correspondingly, the upper rackassembly 80 and lower rack assembly 90 must be in substantialalignment). Second, as shown in FIG. 6, the distance “D” (measured thesame way as “P”, i.e., centerline-to-centerline) between the lowermostgear tooth 102 of the upper rack 82 and the uppermost gear tooth 112 ofthe lower rack 92 must be equal to a multiple of the predetermined pitch“P”, in other words, P, 2P, etc. Most preferably, and for the smoothesttranslation, D is equal to P. As shown in FIG. 6, there is preferably aspace “S” between the upper rack 82 and the lower rack 92 so that theupper rack 82 and lower rack 92 do not abrade one another due tovibrations present during normal operation of the cartridge handlingassembly 50 (FIGS. 1 and 2). For example, while the racks 82, 92 mayeach have a pitch “P” of approximately 6 mm, the spacing “S” may beapproximately 0.5 mm.

[0033] The second cartridge handling assembly 51 may comprise a piniongear assembly 60 identical to that described above in association withthe first cartridge handling assembly 50.

[0034] As described above, any number of modular units 12, 13, 14 may bestacked vertically upon one another to form adjacent modular units of amodular data storage system 10, FIG. 1. The rack assemblies (e.g. 80,90) may be aligned using an alignment tool (not shown) and shiftingeither the lower adjacent modular unit 13 or the upper adjacent modularunit 12 until the rack assemblies are aligned. Then, the lower modularunit 13 may be fixedly attached to the upper modular unit 12 using anyconventional attachment means such as screws or the like. The distance“D” and spacing “S” are maintained between the upper and lower racks 82,92 (and upper and lower rack assemblies 80, 90) because of the positionof the racks 82, 92 within each modular unit housing 30, 32.

[0035] In an alternative embodiment shown in FIG. 7, the modular units212-217 of the modular data storage system 210 are placed within aconventional rack mount housing 220. The housing 220 may comprise aplurality of attaching devices 222, such as the L-shaped brackets shownin FIG. 7, which are adapted to fixedly attach the modular units 212-217to the housing 220. The attaching devices 222 may be fixedly attached tothe rack mount housing 220 using a screw or the like. Each of themodular units 212-217 may be inserted into the rack housing 220 and thenfixedly attached to several of the attaching devices 222 screws or thelike. It is to be understood that, while L-shaped brackets are shown inFIG. 7, any conventional attaching device may be used to attach themodular units to the rack mount housing, as long as the attaching devicedoes not interfere with the vertical translation of the cartridgehandling assemblies 50, 51 among the adjacent modular units. The modularunits 212-217 may alternatively be directly attached to the rack housing220. The size of the rack mount housing 220 (and number of attachingdevices 222 the housing 220 can accommodate) may be determined inaccordance with a customer's requirements. The modular units may havefront covers or bezels mounted over the front faces thereof such asdescribed in U.S. patent application Ser. No. 295,491, filed Apr. 20,1999 for BEZEL ASSEMBLY AND METHOD OF PRODUCTION of Wayne E. Foslien,Joseph M. White, and Seiya Ohto, now U.S. Pat. No. 6,183,053, issuedFeb. 6, 2001, which is hereby specifically incorporated by reference forall that is disclosed therein.

[0036] It can be seen from the above description that each of themodular units may have many common components, such as the housing,racks, drives and magazines. In addition, identical redundant cartridgehandling assemblies 50, 51 may be used, one at a time in alternativeoperating modes, to access the drives and magazines of all of theintermediate modular units (e.g. 13, or 213-216), as well as any furtherintermediate unit that may be subsequently added. This replication ofcomponents provides many advantages to the manufacturer as well aslow-end, mid-range, and high-end customers, since a single set ofcomponents could be used to supply an entire family of data storagesystems. FIG. 7 shows operation in the fall back operating mode afterpicker assembly 51 has failed and has descended due to the force ofgravity into housing unit 217. Picker 50 is shown operating to movecartridge receiving devices mounted in intermediate housing units213-216.

[0037] Having thus described two specific data storage systemembodiments 10 and 210, operation of each will now be explained withreference to a data storage system 300 shown schematically in FIGS.8-10.

[0038] As best shown by FIG. 8, the data storage system 300 comprises acartridge receiving portion 308 having a plurality of cartridgereceiving devices 310, 312, 314, 316, 318, 320 therein. A first pickerstorage bay 330 is mounted at a first end 332 of the cartridge receivingportion 308. A second picker storage bay 340 is mounted at a second end342 of the cartridge receiving portion 308. An unobstructed pickerdisplacement path 350, which may be a vertical shaft having a centrallongitudinal axis XX extends through the cartridge receiving portion 308and has opposite first and second terminal ends 352, 354 defined by thefirst and second picker receiving bays 330, 340.

[0039] First and second cartridge handling assemblies or pickers 360,362 are displaceably mounted in the picker displacement path 350. Thecartridge receiving devices 310, 312, 314, etc. are arranged invertically stacked layers defining tiers within the cartridge receivingportion 308 of the data storage system 300. The picker bays 330, 340also define top and bottom tiers of the data storage system whichcomprises first, second, third, fourth and fifth tiers 381, 382, 383,384, 385. The tiers may each be associated with a separate housingmodule such as shown in FIGS. 1 and 7 or, alternatively, may all beprovided in a single non-modular housing.

[0040] In a pre-start up operating mode a second picker 362 withfeedback unit 361 is positioned in the second picker storage bay 340 anda first picker 360 with feedback unit 359 is positioned in the fourthtier 384, resting on top of the second picker, FIG. 8. This willtypically be the position of the pickers when the data storage system300 is in storage or when it is shipped to a customer. Each picker 360,362 is operably connected such as by Z-fold ribbon cable, to a centralcontrol unit 400, FIG. 11, which receives a position feedback signaltherefrom such as by an associated motor encoder 359, 361 or the like.Control unit 400 sends control signals to assemblies 360, 362 to actuateeach associated drive unit to vertically displace each handling assemblyto a desired position.

[0041] At system start up the first picker 360 is driven by anassociated drive assembly, such as described above with reference toFIG. 2, to a location inside the first picker storage bay 330 as shownin FIG. 9. It then remains in this position during normal operation ofthe data storage system 300. In one implementation the first picker 360remains in a power-on state at all times after system start up and ismaintained at the position shown in FIG. 9 by force applied by itsassociated vertical lift drive assembly. In another implementation agate or lock device 364 is mounted at a lower portion of the firstpicker storage bay 330 and allows the first picker to pass it withoutresistance on the way up 370 but prevents the picker from moving down372 once it has entered the bay 330. ( However the gate 364 may beselectively tripped by a gate release device 366 to enable the picker360 to leave the bay 330 to commence a fall back operating mode asdescribed below.) After system startup the second picker assembly 362has access to all of the cartridges (not shown) received in thecartridge receiving devices 312, 314, 316, etc in tiers 382-384 and mayselectively move cartridges between the various receiving devices.

[0042] If the second picker 362 malfunctions and loses power it willdescend, due to gravity, into the second picker storage bay 340, asshown in FIG. 10. A sensing assembly 361 within the system, such as adrive motor encoder or current sensor senses the loss of power to thesecond picker 362 and in response thereto actuates the first picker 360for required cartridge handling operations of the system. In theimplementation in which a gate 364 is used to hold the first picker inits storage bay during normal mode operation, the gate tripper 366 isactuated after power loss to the second picker is sensed. The tripperthus releases gate 364 allowing the now powered first picker 360 toleave storage bay 330 and commence operation in the fall back mode. Inthe implementation wherein the first picker 360 is constantly powered nosuch gate or gate release mechanism needs to be actuated.

[0043] The second storage bay 340 is preferably constructed and arrangedso that the second picker assembly 362 may be readily removed therefromwhile the system 300 is operating in the fall back mode. The secondpicker assembly 362 may thus be replaced by another picker assembly ormay be removed, repaired and reinstalled so that the system 300 may bereturned to the above described normal mode of operation with aredundant picker.

[0044] Although an implementation of system 300 has been described abovein which shaft 350 is vertical, it is to be understood that system 300could also be implemented in a configuration identical to that describedabove in which shaft 350 is inclined or positioned horizontally. In suchconfigurations gravity may not act on picker 362, or may be insufficientto move it into bay 340 when it loses power. However, the first picker360 can be used, in an initial operation in the fall back operatingmode, as a pusher device to push picker 362 into bay 340. Alternativelypicker 362 could be provided with a small rechargeable battery operatedbackup drive motor 390, or other drive device, which is actuated uponfailure of the main drive assembly to drive the second picker assembly362 into the second picker storage bay 340.

[0045] It is also to be understood that the cartridge receiving portion308 of the data storage system 300, either including or excluding thepicker storage bays 330, 340 may be implemented as a series of attachedmodular units, such as described with reference to FIGS. 1-7, or as asingle housing unit which is not separable into different modules.

[0046] While illustrative and presently preferred embodiments of theinvention have been described in detail herein, it is to be understoodthat the inventive concepts may be otherwise variously embodied andemployed and that the appended claims are intended to be construed toinclude such variations except insofar as limited by the prior art.

We claim:
 1. A data storage system comprising: a) a housing assemblyhaving a first end and a second end and having an open shaft extendingbetween said first end and said second end and comprising: i) a firsthousing unit having a first portion of said open shaft extendingtherethrough, said first unit being positioned proximate said first endof said housing assembly; ii) a second housing unit having a secondportion of said open shaft extending therethrough, said second unitbeing positioned proximate said second end of said housing assembly;iii) at least a third housing unit having a third portion of said openshaft extending therethrough having a plurality of data media receivingdevices mounted therein, said third housing unit being disposed betweensaid first and second housing units; b) a first media handling assemblydisplaceably positioned within said open shaft; and c) a second mediahandling assembly displaceably positioned within said open shaft.
 2. Thedata storage system of claim 1, said data storage system having a firstoperating mode wherein said first media handling assembly isstationarily positioned within said first portion of said shaft and saidsecond media handling assembly is positioned in said shaft in other thansaid first portion.
 3. The data storage system of claim 2, said datastorage system having a second operating mode wherein said second mediahandling assembly is stationarily positioned within said second portionof said shaft and said first media handling assembly is positioned insaid shaft in other than said second portion.
 4. The data storage systemof claim 3 wherein, in said first operating mode, said second mediahandling assembly is positioned in said shaft in other than said firstportion and said second portion and wherein, in said second operatingmode, said second media handling assembly is positioned in said shaft inother than said first portion and said second portion.
 5. The datastorage system of claim 1, said data storage system having a pre-startupmode wherein said second media handling assembly is positioned in saidsecond portion of said shaft and said first media handling assembly ispositioned in said shaft next adjacent to said second media handlingassembly.
 6. The data storage system of claim 4, said data storagesystem having a pre-startup mode wherein said second media handlingassembly is positioned in said second portion of said shaft and saidfirst media handling assembly is positioned in said shaft next adjacentto said second media handling assembly.
 7. The data storage system ofclaim 6 comprising a plurality of housing units disposed between saidfirst and second housing units and having portions of said open shaftextending therethrough and having a plurality of data media receivingdevices mounted therein.
 8. A data storage system comprising: a) a mediapicker displacement path having a first end and a second end; b) a firstmedia picker assembly mounted in said picker displacement path; c) asecond media picker assembly mounted in said picker displacement path;d) a plurality of data storage media access locations located along saidpicker displacement path; e) said first media picker assembly beingdisplaceable along said path from said first end to a point ofinterfering contact with said second media picker assembly; f) saidsecond media picker assembly being displaceable along said path fromsaid second end to a point of interfering contact with said first mediapicker assembly.
 9. The data storage system of claim 8, said storagesystem having a pre-startup operating mode wherein said first mediapicker assembly is positioned at said first end of said displacementpath and said second picker assembly is positioned next adjacent saidfirst picker assembly.
 10. The data storage system of claim 8, saidstorage system having a normal running operating mode wherein said firstmedia picker assembly is stationarily positioned at said first end ofsaid displacement path and said second picker assembly is selectivelypositionable along the remainder of said path.
 11. The data storagesystem of claim 10, said storage system having a fall back runningoperating mode wherein said second media picker assembly is stationarilypositioned at said second end of said displacement path and said firstpicker assembly is selectively positionable along the remainder of saidpath.
 12. The data storage system of claim 11, said storage systemhaving a transition mode of operation occurring between said normalrunning mode and said fall back mode wherein said second picker assemblymoves from an intermediate position in said path to said second end ofsaid path.
 13. A method of operating a data storage library comprising:moving data storage media between media holding positions located alonga one-picker-at-a-time picker displacement path with a second mediapicker assembly until the second media picker assembly malfunctions;after it malfunctions moving the second picker assembly a to a secondend of the picker displacement path; maintaining a first media pickerassembly at a stationary position at a first end of the pickerdisplacement path until the second picker assembly malfunctions; afterthe second picker assembly malfunctions moving data storage mediabetween media holding positions located along the picker displacementpath with the first picker assembly.
 14. The method of claim 16 whereinmoving the second picker assembly to a second end of the pickerdisplacement path comprises moving the second picker assembly with theforce of gravity.
 15. The method of claim 16 wherein moving the secondpicker assembly to a second end of the picker displacement pathcomprises moving the second picker assembly with an auxiliary drive unitmounted on the second picker assembly.
 16. The method of claim 16wherein moving the second picker assembly to a second end of the pickerdisplacement path comprises moving the second picker assembly with thefirst picker assembly.